Definition
A straight, unobstructed path between a transmitter and a receiver. Radio signals at VHF and higher frequencies (including DME) travel in essentially straight lines and cannot bend around the curvature of the earth or pass reliably through terrain, so the aircraft and the ground station must have a direct line between them for the signal to be usable.
Plain English
A straight, clear path between two points with nothing solid in the way. For radios that work this way, if a mountain or the curve of the earth is between you and the station, the signal can't get through.
Context Anchor
Seen in discussions of Distance Measuring Equipment (DME), navigation radio reception, and why some signals become unreliable at low altitude or behind terrain.
Derivation
From the everyday idea of seeing something in a straight line without obstacles; applied to radio waves that also travel in straight lines.
Why Pilots Care
DME distance information is lost or erroneous once the signal path is blocked by terrain or the horizon.
Analogy
It is similar to trying to shine a flashlight at someone across uneven ground. If a hill is between you and the other person, the light may not reach them even though the flashlight is working.
Intuition Check
Line-of-sight does not mean the pilot must visually see the station. It means the aircraft antenna needs a reasonably clear radio path to the ground antenna.
Example Sentence 1
Because DME is line-of-sight, the signal was lost as the aircraft descended below the ridge.
Example Sentence 2
At low altitude the radio horizon limited line-of-sight and reduced usable DME range.